The present invention relates to a method of cleaning an exhaust gas discharged from internal combustion engines such as automobile engines, etc. and an apparatus for cleaning such an exhaust gas, and more particularly to a method of cleaning an exhaust gas of diesel engines, etc. by efficiently reducing nitrogen oxides in the exhaust gas, and an apparatus for conducting such an exhaust gas-cleaning method.
Recently, environmental pollution has become a serious problem in a global scale, and among them, air pollution such as photochemical smog and acidic rain caused by nitrogen oxides such as nitrogen monoxide, nitrogen dioxide (hereinafter referred to simply as "NOx") in the air is a serious problem. NOx is generated from automobile engines such as diesel engines, and large combustion apparatuses such as cogenerators. Also, fun heaters and other combustion equipments which are widely used at home are sources of nitrogen oxides.
In the case of exhaust gas from gasoline engines, NOx is usually removed by using so-called three-component catalysts. Also, in the case of large, stationary combustion apparatuses such as internal. combustion engines for cogenerators, metal oxide catalysts such as V.sub.2 O.sub.5 are used, and ammonia is introduced into exhaust gas, whereby nitrogen oxides in the exhaust gas are catalytically and selectively reduced.
However, in the case of an exhaust gas having a relatively high oxygen concentration such as those discharged from diesel engines and those discharged from gasoline engines operable in a lean state, efficient removal of NOx cannot be achieved with the above-described three-component catalysts suitable for usual gasoline engines. Also, the reduction of NOx with ammonia introduced into an exhaust gas cannot be applied to movable exhaust gas sources such as automobiles, because this reduction system needs large apparatuses and poisonous, expensive ammonia.
Therefore, it has been desired to develop a method of reducing NOx in the exhaust gas having a relatively high oxygen concentration which is discharged from diesel engines, etc., and various attempts have been made so far.
For instance, there have been proposed methods of reducing NOx in an exhaust gas with a hydrocarbon introduced into the exhaust gas. One example of such methods is disclosed by Japanese Patent Publication No. 44-13002, which comprises passing an exhaust gas through a honeycomb-type ceramic filter carrying a platinum-group metal catalyst while controlling the temperature and flow rate of the exhaust gas, and adding a gaseous reducing fuel (specifically, methane, etc.) to the exhaust gas.
However, this method fails to efficiently reduce NOx in the exhaust gas discharged from diesel engines, etc. According to research by the inventors, even though a hydrocarbon having a small carbon number which is in a gas state in a normal condition, such as methane, propane, etc. is added as an NOx-reducing agent to the exhaust gas discharged from diesel engines, a large removal ratio of NOx cannot be obtained.
There has also been proposed a method of cleaning an exhaust gas, which comprises mixing an exhaust gas containing oxygen and NOx with a hydrocarbon to cause a reaction between oxygen and a hydrocarbon such that the hydrocarbon is partially oxidized to a reducing hydrogen gas and carbon monoxide and to lower the oxygen concentration, the resulting hydrogen gas and carbon monoxide being reacted with NOx in the exhaust gas, thereby decomposing them to nitrogen, carbon dioxide and water (Japanese Patent Laid-Open No. 49-122474). However, since the reduction reaction of NOx should be conducted at a relatively high temperature in this method, it is not suitable for cleaning the exhaust gas of automobiles.
There is an alternative method for reducing NOx which comprises adding a petroleum fuel as a reducing agent to a combustion exhaust gas in a high-temperature region alone or together with part of the combustion exhaust gas with or without air, and adding an air to the combustion exhaust gas in the downstream to reduce NOx in the combustion exhaust gas, the addition of the petroleum fuel such as methane, propane, gasoline, kerosine, naphtha, heavy oil, etc. being conducted by a plurality of steps such that a ratio of the amount of oxygen remaining in the exhaust gas to the amount of oxygen necessary for completely burning the petroleum fuel added is within a particular range (Japanese Patent Laid-Open No. 54-79161).
However, this method cannot remove NOx efficiently without keeping a region where a reducing agent and NOx are reacted at 1000.degree. C. or higher. Thus, this method is also not suitable for cleaning the exhaust gas of automobiles.
There has been proposed a method of reducing NOx by using a catalyst such as zeolite and Al.sub.2 O.sub.3, etc. containing copper, and adding a hydrocarbon having a small carbon number to the exhaust gas (SAE Technical Paper 900496, 1990, and Japanese Patent Laid-Open No. 63-100919). However, catalysts used in this method are poor in heat resistance, durability and low-temperature characteristics, and the method using such catalysts fails to achieve a high reduction ratio of NOx at a low temperature and when the exhaust gas has a high oxygen concentration. Accordingly, it cannot be used for practical purposes.